The present invention relates to an objective-lens driving apparatus for use in an optical disk apparatus for recording information on the recording surface of an optical disk or for reading the information recorded thereon.
In an optical disk apparatus for recording information on a disk-shaped recording medium or for reading and reproducing the information recorded thereon, an objective-lens driving apparatus is an apparatus for driving an objective lens in the focusing direction (i.e., the direction in which the objective lens comes closer to/goes away from the recording surface of the optical disk) and in the tracking direction (i.e., the radial direction of the disk). Here, the objective lens light-converges light on the optical-disk recording surface. Generally, the objective-lens driving apparatus incorporates a movable unit with the objective lens, a supporting member for supporting this movable unit, and a magnetic circuit with a yoke and a permanent magnet. A focusing coil and a tracking coil are mounted onto the movable unit. Applying a driving current to the focusing coil drives the movable unit in the focusing direction by an electromagnetic force generated by the interaction with a magnetic flux from the permanent magnet. Similarly, applying the driving current to the tracking coil drives the movable unit in the tracking direction by an electromagnetic force generated by the interaction with the magnetic flux from the permanent magnet.
In the objective-lens driving apparatus like this, if the objective lens has been inclined, an optical aberration occurs, thereby enlarging a light-converged spot. This makes it impossible to correctly record the information on the disk, or results in a degradation in the reproduced signal.
Conventionally, there has been known the optical pick-up unit which was devised in order to suppress this inclination of the objective lens (e.g., JP-A-2001-101687). This optical pick-up unit incorporates the tracking coil, the focusing coil, the objective lens, a lens holder, a damper base for supporting the lens holder in a movable manner via plural suspension wires, the yoke, and the permanent magnet. Moreover, in the optical pick-up unit, the configuration size of the permanent magnet is set so that electromagnetic forces, which exert themselves on the tracking coil and/or the focusing coil thereby to cause the optical axis of the objective lens to be inclined from its reference axis, will substantially cancel out each other.
In the above-described related art, the configuration size of the permanent magnet is set at a certain value. This setting has canceled out moments generated at the focusing coil and the tracking coil when the objective lens is displaced, thereby suppressing the inclination of the objective lens. This setting, however, imposes the restrictions on the sizes of the permanent magnet, the focusing coil, and the tracking coil. As a consequence, the design's degree-of-freedom has been limited, and there has been acquired only effect that is not necessarily sufficient in an aspect of the apparatus's downsizing.